Heat shock or stress dramatically increases cellular production of several classes of highly conserved chaperone proteins, commonly known as heat-shock proteins (HSPs). These chaperones, including the members of the HSP60, HSP70, and HSP90 families, are ATP-dependent molecules that facilitate/ensure proper client protein (e.g. protein that requires interaction with the chaperones for its activity and stability) folding, prevent non-specific aggregations, and maintain active protein conformations.
The HSP90 family, comprised of HSP90 α and β, Grp94 and TRAP-1, represents one of the most abundant cellular proteins, accounting for 1-2% of total protein in a mammalian cell under normal conditions. HSP90 is unique among cellular chaperones in that it is not required for general co-translational protein folding but is instead dedicated to a unique set of cellular proteins, many of which are key signaling molecules critically involved in cell growth, differentiation, and apoptosis. So far over 100 proteins have been documented to associate with HSP90 and this list of client proteins is expanding rapidly.
Crystallographic studies have revealed the existence of an unconventional low affinity ATP binding cleft at their N-terminal domain that is well conserved among the four HSP90 family members. ATP binding and hydrolysis play an essential role in the regulation of chaperone functions. The occupancy of the ATP binding site by the ansamycin antibiotics geldanamycin (GM) and herbimycin A (HA), as well as the structurally unrelated fugal metabolite radicicol, inhibits the intrinsic ATPase activity of HSP90 and blocks the ATP/ADP-regulated association-dissociation cycles between HSP90 and client proteins. Consequently, ATP-competitive HSP90 inhibitors induce destabilization and eventual ubiquitin-dependent degradation of client proteins.
HSP90 has generated tremendous interest as a novel anti-cancer target following the realization that many of its clients are bona fide oncoproteins that are frequently overexpressed, mutated, or constitutively active in tumor cells. These include well known and established cancer drug targets such as receptor tyrosine kinases (HER-2/neu, epidermal growth factor receptor EGFR, Met and insulin-like growth factor-1 receptor IGF-1R), metastable serine/threonine kinases (Akt and Raf-1), mutated signaling proteins (Flt3, v-Src), chimeric oncoproteins (Bcr-Abl, NPM-ALK), cell-cycle regulators (CDK4 and CDK6), transcription factors (estrogen and androgen receptors ER and AR, hypoxia-inducible factor HIF-1α) and apoptosis regulators (Survivin and Apaf-1). It is notable that HSP90 client proteins functionally contribute to all of the six “hallmarks of cancer”, which include (with examples of relevant HSP90 client proteins in parenthesis) 1) self-sufficiency in growth signals (ErbB2, Raf-1), 2) insensitivity to growth suppression signals (Plk, Myt1), 3) evasion of apoptosis (Akt, RIP), 4) acquisition of limitless replicative potential (hTERT), 5) sustained angiogenesis,(HIF-1α, FAK) and 6) invasion and metastasis (Met). The association with HSP90 ensures that these otherwise unstable oncoproteins function properly in multiple signaling pathways that are essential in maintaining the unregulated growth and the malignant phenotypes of tumors.
Inactivation of HSP90 by an ATP-competitive inhibitor will induce simultaneous depletion of multiple oncoproteins and cause concurrent inhibition of various oncogenic signaling pathways. Therefore, by disrupting the function of a single molecular entity HSP90, an HSP90 inhibitor may uniquely provide a combinatorial attack on multistep oncogenesis and block all of the six hallmarks of cancer. Depending on cellular contexts, HSP90 inhibitors effectively cause growth arrest, differentiation, or apoptosis of tumor cells both in vitro and in vivo. In addition, HSP90 itself is overexpressed (about 2-20 fold) in multiple tumor types as a result of oncogenic transformation (e.g. accumulation of mutated proteins) and cellular stress (e.g. low pH and lack of nutrients). Overexpression of Hsp90 has been shown to correlate with poor prognosis in breast cancer.
Cancer cells are highly adaptive to hostile microenvironments and are capable of acquiring drug resistance, in part due to their inherent genetic instability and plasticity. Moreover, most forms of cancer are polygenic and harbor multiple signaling aberrations. HSP90 may be a key component of the very machinery that allows certain cancer cells to escape apoptotic death and evoke alternative or overlapping signaling to efficiently develop resistance to a specific drug treatment. Consequently, inhibition of Hsp90 by concurrently disrupting a wide range of oncogenic pathways may prove to be a very effective approach to combat a variety of hard-to-treat tumor.20-23 The cancers include, for example, breast cancer1, ovarian2, prostate3, chronic myelogenous leukemia (CML)4, melanoma5, gastrointestinal stromal tumors (GISTs)6, master cell leukemia7, testicular tumor7, acute myelogenous leukemia8,9, gastric tumor10, lung11, head and neck12, glioblastoma13, colon14, thyroid15, stomach, liver, multiple myeloma16, renal17, and lymphoma18,19.
In addition to cancers, Hsp90 inhibitors may also have the potential to treat non-oncological indications where diseased cells show increased expression and usage of HSP90. These include, but are not limited to viral diseases mediated by hepatitis B virus (HBV), hepatitis C virus (HCV) and herpes simplex virus type 1 (HSV-1) as well as autoimmune diseases including those mediated by persistent lymphocyte activation. In all these cases, elevated Hsp90 activity either facilitates virus assembly and replication or is required for aberrant signaling transduction in inappropriately activated lymphocyte. Furthermore, HSP90 inhibitors are also known to induce upregulation of other heat shock proteins (e.g. HSP70), which may offer neuroprotection and cardioprotection against ischemic injury as well as damages caused by protein-aggregation. Therefore, HSP90 inhibitors offer therapeutic potential in treatment of central nervous system (CNS) disorders and cardiovascular diseases.